Compaction Control and the Index Unit Weight

Procedures for measurement of percent compaction and several related practical issues are discussed. When measuring percent compaction, the “maximum” unit weight should be determined for each field unit weight test or nuclear unit weight measurement because minor differences in gradation and grain shape that are neither visually observable nor apparent from conventional gradation tests can have a significant effect on the “maximum” unit weight. A one-point compaction test should be performed as a minimum in most cases to reduce errors and the number of tests needed, to help focus on the soil type, and to help reduce conflicts with contractors.

It would be very desirable to standardize on the use of only one compaction test for all types of construction control. Use of multiple compaction tests in specifications for various fill types causes confusion in practice without adding any complementary benefit.

ASTM Test Method for Moisture-Density Relations of Soils and Soil-Aggregate Mixtures Using 10-lb (4.54-kg) Rammer and 18-in. (457-mm) Drop (D 1557) is the test preferred by the author for controlling fill in dam embankments, building foundations, highways, and other fills and for establishing the degree of compaction of natural soils. Since the “maximum” unit weight is not a true maximum, it is suggested that the value measured with the selected standardized test be referred to as the index unit weight (IUW).

Field unit weight tests for compaction control should be done within one layer, below surface disturbance, and in representative locations. Compaction test procedures for measurement of the index unit weight of clean sands and a discussion of the effects of grain breakage are presented.

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